In this talk we will explore the evolution of both the Internet's most 
heavily used transport protocol, TCP, and the current network 
environment with respect to how this evolving network ultimately impacts 
end-to-end protocols.  The traditional end-to-end assumptions about the 
Internet as increasingly challenged by the
introduction of intermediary network elements (middleboxes) that 
intentionally or unintentionally prevent or alter the behavior of 
end-to-end communications. We will show measurement results showing the 
impact of the current network environment on a number of traditional and 
proposed protocol mechanisms (e.g., Path MTU
Discovery, Explicit Congestion Notification, etc.).  In addition, we 
investigate the prevalence and correctness of implementations using 
proposed TCP algorithmic and protocol changes (e.g., selective
acknowledgment-based loss recovery, congestion window growth based on 
byte counting, etc.).  We present results of measurements taken using an 
active measurement framework to study web servers and a
passive measurement survey of clients accessing information from our web 
server.  We analyze our results in the context of gaining further 
understanding of the differences between the behavior of the Internet in 
theory versus the behavior we observed through measurements.  In 
addition, these measurements can be used to guide the definition of more 
realistic Internet modeling scenarios.